A combination of `RemoteCommand` and `RequestTTY` should do the trick:

    Host <host>
        ...
        RequestTTY yes
        RemoteCommand tty

The problem is that this will mess up your one-off command invocations of ssh, git ssh connections, etc:

    $ ssh <host> ls
    Cannot execute command-line and remote command.

However, we can use `Match` blocks to get around that! There are a thousand ways to skin this cat, but one way is to use an environment variable, here `t` for "tmux". Put this at the end of your config:

    Match exec "test ${t:-0} = 1"
        RequestTTY yes
        RemoteCommand tmux has-session && tmux attach-session || tmux

and the following Just Work, reattaching tmux sessions as necessary:

    $ t=1 ssh <host>
    $ ssh <host> ls